As explained in U.S. Pat. No. 4,750,877 there is a need in the art of moulding plastic products to be able to mould a plastic part having a relatively small diameter hole extending therethrough.
Especially in the field of medical products, such as catheters, it is important to exert great care in maintaining the constant diameters of both the outer surface and the central hole extending through a catheter product since this has the effect of maintaining a substantially straight shank in the finished tubular product. Also, the product must be constructed to have a consistent flex pattern and avoid “kinking” when bent or flexed. Often, such moulded plastic parts are required to have a thin wall of necessarily constant thickness with a longitudinally extending channel extending therethrough.
Previously, especially in the art of injection moulding, when plastic is melted, and forced to flow into a cavity about a fine, small diameter core, the pressure of injection forcing the melt into the mould cavity if not adequately balanced during the moulding operation tends to cause the thin diameter core to be displaced from its substantially centred, straight line configuration relative to the central axis of the mould cavity. This displacement is of course undesirable since a straight line configuration of the central hole as well as the shank portion of the product is required. Such displacement of the core pin relative to the interior surface of the mould cavity is also frequently caused by what may be considered an imbalance in the flow characteristics of the hot melt as it enters the mould cavity and passes along the length thereof in substantially surrounding relation to the core pin. Such imbalance may be caused by a number of factors but also has the tendency to cause uneven forces to be exerted on the core pin tending to cause its displacement from the aforementioned and preferred straight line configuration. For these reasons, the prior art has generally been limited to relatively short tubular lengths in relation to the diameter of the hole through it. If, for example, the tubular product is to be relatively long, which is quite common in the medical industry, a small hole, especially a hole in a thin walled tube is extremely difficult to make with any great degree of accuracy. In addition, different materials provide different problems.
Another problem generally associated with the injection moulding technique of products of the type referred to above is the breakdown in the intensity of the plastic material utilized to form the tubular product when the material is forced to travel along what may be referred to as a circuitous path from the point of leaving the injection nozzle to the point of entering and passing along the length of the mould cavity. This is particularly true when the plastic material, after being melted, is forced to flow along a circuitous path or a path including one or more right angles at a relatively high speed. Attempts to slow the speed of injection of the inflowing melt however have met with little success especially in the formation of thin walled tubular products. This is primarily due to the fact that the melt rapidly cools and therefore solidifies as it flows along the length of the mould cavity especially at slow speeds. Accordingly, when using injection moulding techniques at such slow speeds, attempts have been made to raise the temperature of the plastic material to a somewhat higher than normal temperature, prior to injection to overcome the problems of the prior art. This higher temperature results in a degradation of many plastic materials which has obvious disadvantages in the making of thin walled tubular products.
U.S. Pat. No. 2,443,053 discloses a method and an apparatus for manufacturing hollow cylindrical plastic articles with a moving core, which is moved during the injection of moulding material through an annular inlet in the stationary outer moulding member. However, according to the moulding technique disclosed herein, the first end section of the article is formed whereafter a tubular section of the article is formed. However, there are limitations to which designs the article can have when moulded according to this method as only the front end of the article can be provided with a special design as the rest of the article must have a cylindrical shape.
From U.S. Pat. No. 7,871,261 and U.S. Pat. No. 7,910,044 a method and an injection moulding apparatus for producing profiled elongated articles are disclosed where a moving inlet is provided for a moving moulding core.
From EP 1 116 567 there is known a gas assisted injection moulding technique. However, this technique only allows making short catheters.